Bisphenol A [2,2′-bis(4-hydroxyphenyl)propane] has been used as a starting material for producing a variety of polymers. In recent years, there has especially been a high demand for aromatic polycarbonates having excellent impact resistance and transparency, and bisphenol A having low coloring has been desired for producing aromatic polycarbonates.
Bisphenol A is usually produced by reacting phenol with acetone in the presence of homogenous acids or solid acid catalysts. The reaction mixture includes unreacted acetone, unreacted phenol, water produced during the reaction, and other side-products in addition to bisphenol A. The main component of the side-products is 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane (hereinafter, referred to as “o,p′-BPA”), and in addition, the side-products include trisphenol, a polyphenol compound, a chroman compound, colored impurities and the like.
Bisphenol A is obtained by cooling the reaction mixture to form a slurry containing a crystalline adduct of phenol-bisphenol A (hereinafter, referred to as “adduct crystals”), if necessary, after removing unreacted acetone and water produced during the reaction and concentrating bisphenol A [a crystallization step], separating the adduct crystals from the slurry [a solid-liquid separation step], washing the separated adduct crystals with phenol, etc. [a washing step], and then removing phenol by means of distillation or stripping.
As described above, the reaction mixture of bisphenol A contains colored impurities. In the steps of producing bisphenol A, the main step of removing the colored impurities includes the crystallization step, the solid-liquid separation step, and the washing step. The purity and the hue of the adduct crystals produced in the crystallization step give an effect on the purity and the hue of the bisphenol A product. Further, when the adduct crystals produced in the crystallization step are fine crystals, the amount of a mother liquor adhered to the surface of crystals per unit weight of crystals is increased. There is also deterioration in the separation efficiency of colored impurities in the solid-liquid separation step and the washing step subsequent thereto. Accordingly, in order to produce bisphenol A, it is important to produce adduct crystals having high purity and good hue and generating a small amount of fine crystals in a crystallization step.
As a method of producing adduct crystals with high purity and excellent hue, there can be mentioned a method comprising, by using a series of multi-stage crystallization tanks, sequentially cooling to the targeted final temperature, as described in Japanese Unexamined Patent Application Publication No. 5-117191, Japanese Unexamined Patent Application Publication No. 7-258131, and PCT Japanese Translation Patent Publication No. 2003-528840. However, this method does not disclose the ratio (=B/A[−]) of a solid fraction B [% by weight] in the final crystallization stage to a solid fraction A [% by weight] in the first crystallization stage, which is required to obtain adduct crystals having high purity as disclosed in the present invention. Further, this method is insufficient for producing adduct crystals having high purity, good hue, and a small amount of fine crystals.
Japanese Unexamined Patent Application Publication No. 5-117191 discloses a method comprising, by using n-number of crystallization towers having an inner cylinder provided with an inlet port at its upper portion, discharging a part of the slurry of adduct crystals in the crystallization towers, cooling the discharged slurry in a heat exchanger provided at the outside of the crystallization towers, and then recycling the slurry of adduct crystals into the crystallization tower, and at the same time at least a part of the slurry of adduct crystals in the n-th stage is heated to dissolve fine crystals and is then recycled into the crystallization tower. However, since the adduct crystals are easily crushed, the adduct crystals are crushed during circulation of the slurry, which inevitably leads to the formation of fine crystals. In addition, it is necessary to heat the slurry of adduct crystals having been once cooled, thereby causing energy loss.
Japanese Unexamined Patent Application Publication No. 7-258131 discloses a method in which, by using an n-stage cascade of crystallization tanks connected in series with the number of crystallization reactors n (where n>1), the reaction mixture is circulated at a circulation rate of at least 500 m3/h in a state where the residence time in each crystallization device is set to three hours or more. In this method, a plurality of crystallization devices which has a residence time of at least 3 hours is required, thereby increasing the cost for their equipment. Further, since a circulation rate of 500 m3/hr or more is required, a great amount of power is required. Furthermore, in the case where the circulation of the slurry is performed at such a large circulation rate, the amount of fine crystals produced by crushing the adduct crystals becomes large.
PCT Japanese Translation Patent Publication No. 2003-528840 discloses a process of producing adduct crystals using crystallization devices of one to five stages during a residence time of 2 to 12 hours, by using one or more crystallization devices having a crystallization tank, a circulating pump and a cooler. In this method, each crystallization device also needs a circulating pump and the adduct crystals are thus crushed by the circulating pump, thereby forming fine crystals. In addition, the hue of bisphenol A obtained by removing phenol from the adduct crystals becomes insufficient.
As such, in the conventional known method comprising sequentially cooling the reaction mixture to the targeted final crystallization temperature by using a series of multi-stage crystallization tanks, the ratio (B/A) of a solid fraction B in the final crystallization stage to a solid fraction A in the first crystallization stage which is required to obtain adduct crystals having high purity is not disclosed. In addition, from the view point of equipment investment, it is known that the crystallization devices practically having three or less crystallization stages and preferably two crystallization stages are preferable. In such method, there is a problem that it is difficult to precisely control the temperature according to a growth process of crystals, since the temperature of the slurry in one crystallization device is approximately homogenous. Therefore, it is not possible to precisely control the temperature according to a growth process of crystals.
The present invention relates to a process of producing bisphenol A. More particularly, it is an object of the present invention to provide a process of producing bisphenol A having excellent economical efficiency, high purity and an excellent hue.